There have been two outbreaks of human poisoning by polychlorinated biphenyls (PCBs) and their thermal breakdown products; the first, in Japan in 1968, the second in Taiwan in 1979. Because PCBs are a world wide pollution problem, these episodes have been studied carefully, since they have presented the only opportunity to observe directly the toxicity of PCBs in human beings outside the workplace. Laboratory methods for the evaluation of these outbreaks were relatively unsophisticated in 1968; there has been great progress in analytical methods since. In collaboration with Taiwanese scientists, the Epidemiology Branch, NIEHS, had the opportunity to examine over 100 children who had been in utero at the time of the 1979 poisoning or afterward. These children continued to be affected, since the chemicals cannot be excreted from the mother's body. In a collaborative effort, the mass spectrometry workgroup will analyze approximately 120 serum samples for tetra-, penta- and hexachlorodibenzofurans at the part per trillion level. At the present time extraction procedures are being validated by RTI in preparation for the analysis of the samples.